Pterygotidae
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Pterygotidae (the name deriving from the type genus ''
Pterygotus ''Pterygotus'' is an extinct genus of giant predatory eurypterid, a group of extinct aquatic arthropods. Fossils of ''Pterygotus'' have been discovered in deposits ranging in age from Middle Silurian to Late Devonian, and have been referred to s ...
'', meaning "winged one") is a family of
eurypterid Eurypterids, often informally called sea scorpions, are a group of extinct marine arthropods that form the Order (biology), order Eurypterida. The earliest known eurypterids date to the Darriwilian stage of the Ordovician period, 467.3 Myr, mil ...
s, an extinct group of aquatic
arthropod Arthropods ( ) are invertebrates in the phylum Arthropoda. They possess an arthropod exoskeleton, exoskeleton with a cuticle made of chitin, often Mineralization (biology), mineralised with calcium carbonate, a body with differentiated (Metam ...
s. They were members of the superfamily
Pterygotioidea Pterygotioidea (the name deriving from the type genus ''Pterygotus'', meaning "winged one") is a superfamily (taxonomy), superfamily of eurypterids, an extinct group of aquatic arthropods. Pterygotioids were the most Derived trait, derived membe ...
. Pterygotids were the largest known
arthropod Arthropods ( ) are invertebrates in the phylum Arthropoda. They possess an arthropod exoskeleton, exoskeleton with a cuticle made of chitin, often Mineralization (biology), mineralised with calcium carbonate, a body with differentiated (Metam ...
s to have ever lived with some members of the family, such as ''
Jaekelopterus ''Jaekelopterus'' is a genus of predatory eurypterid Eurypterids, often informally called sea scorpions, are a group of extinct marine arthropods that form the Order (biology), order Eurypterida. The earliest known eurypterids date to the ...
'' and ''
Acutiramus ''Acutiramus'' is a genus of giant predatory eurypterid, an extinct group of aquatic arthropods. Fossils of ''Acutiramus'' have been discovered in deposits of Late Silurian to Early Devonian age. Eight species have been described, five from Nor ...
'', exceeding 2 metres (6.6 ft) in length. Their fossilized remains have been recovered in deposits ranging in age from 428 to 372 million years old (
Late Silurian The Silurian ( ) is a geologic period and system spanning 23.5 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the third and shortest period of t ...
to
Late Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era during the Phanerozoic eon, spanning 60.3 million years from the end of the preceding Silurian period at million years ago ( Ma), to the beginning of the succeeding ...
). One of the most successful groups of eurypterids, the pterygotids were the only eurypterid family to achieve a truly worldwide distribution. Several evolutionary innovations made the pterygotids unique among the eurypterids, with large and flattened
telson The telson () is the hindmost division of the body of an arthropod. Depending on the definition, the telson is either considered to be the final segment (biology), segment of the arthropod body, or an additional division that is not a true segm ...
s (the posteriormost division of the body) likely used as rudders to provide additional agility and enlarged
chelicerae The chelicerae () are the arthropod mouthparts, mouthparts of the subphylum Chelicerata, an arthropod group that includes arachnids, horseshoe crabs, and sea spiders. Commonly referred to as "jaws", chelicerae may be shaped as either articulated ...
(frontal appendages) with claws. These claws were robust and possessed teeth which would have made many members of the group formidable predators. The strange proportions and large size of the pterygotid eurypterids led to the quarrymen who discovered the first fossil remains of the group to give them the
common name In biology, a common name of a taxon or organism (also known as a vernacular name, English name, colloquial name, country name, popular name, or farmer's name) is a name that is based on the normal language of everyday life; and is often con ...
" Seraphims". Studies on the cheliceral morphology and compound eyes of the pterygotids have revealed that the members of the group, despite overall morphological similarities, were highly divergent in their ecological roles. Pterygotid ecology ranged from generalized predatory behaviour in basal members of the group, such as ''
Erettopterus ''Erettopterus'' is a genus of large predatory eurypterid, an extinct group of aquatic arthropods. Fossils of ''Erettopterus'' have been discovered in deposits ranging from Early Silurian (the Rhuddanian age) to the Early Devonian (the Lochkov ...
'', to active apex predators, such as ''
Jaekelopterus ''Jaekelopterus'' is a genus of predatory eurypterid Eurypterids, often informally called sea scorpions, are a group of extinct marine arthropods that form the Order (biology), order Eurypterida. The earliest known eurypterids date to the ...
'' and ''
Pterygotus ''Pterygotus'' is an extinct genus of giant predatory eurypterid, a group of extinct aquatic arthropods. Fossils of ''Pterygotus'' have been discovered in deposits ranging in age from Middle Silurian to Late Devonian, and have been referred to s ...
'', and ambush predators and scavengers, such as ''
Acutiramus ''Acutiramus'' is a genus of giant predatory eurypterid, an extinct group of aquatic arthropods. Fossils of ''Acutiramus'' have been discovered in deposits of Late Silurian to Early Devonian age. Eight species have been described, five from Nor ...
''. Some researchers have suggested that the pterygotid eurypterids evolved in something akin to an "
arms race An arms race occurs when two or more groups compete in military superiority. It consists of a competition between two or more State (polity), states to have superior armed forces, concerning production of weapons, the growth of a military, and ...
" with early vertebrates, that the evolution of heavy armor in the
ostracoderm Ostracodermi () or ostracoderms is an informal group of vertebrate animals that include all armored jawless fish of the Paleozoic Era. The term does not often appear in classifications today because it is paraphyletic (excluding jawed fishes and ...
s could be attributed to pressure from pterygotid predation and that later pterygotid decline could be attributed to subsequent evolutionary trends in fish. This hypothesis is mostly considered as far too simplistic of an explanation by modern researchers. Detailed analyses have failed to find any correlation between the extinction of the pterygotids and the diversification of the vertebrates.


Description

Pterygotid
eurypterid Eurypterids, often informally called sea scorpions, are a group of extinct marine arthropods that form the Order (biology), order Eurypterida. The earliest known eurypterids date to the Darriwilian stage of the Ordovician period, 467.3 Myr, mil ...
s, which occur in strata ranging from
Late Silurian The Silurian ( ) is a geologic period and system spanning 23.5 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the third and shortest period of t ...
to
Late Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era during the Phanerozoic eon, spanning 60.3 million years from the end of the preceding Silurian period at million years ago ( Ma), to the beginning of the succeeding ...
in age, ranged in size from quite small animals, such as '' Acutiramus floweri'' at 20 cm (7.9 in), to the largest known arthropods to have ever lived. Several species reached and exceeded 2 metres in length, the largest known species including ''
Jaekelopterus rhenaniae ''Jaekelopterus'' is a genus of predatory eurypterid, a group of extinct aquatic arthropods. Fossils of ''Jaekelopterus'' have been discovered in deposits of Early Devonian age, from the Pragian and Emsian stages. There are two known species: t ...
'' at 2.5 metres and '' Acutiramus bohemicus'' at 2.1 metres. Like all other
chelicerates The subphylum Chelicerata (from Neo-Latin, , ) constitutes one of the major subdivisions of the phylum Arthropoda. Chelicerates include the sea spiders, horseshoe crabs, and arachnids (including harvestmen, scorpions, spiders, solifuges, tic ...
, and other arthropods in general, pterygotid eurypterids possessed segmented bodies and jointed appendages (limbs) covered in a
cuticle A cuticle (), or cuticula, is any of a variety of tough but flexible, non-mineral outer coverings of an organism, or parts of an organism, that provide protection. Various types of "cuticle" are non- homologous, differing in their origin, structu ...
composed of
protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...
s and
chitin Chitin (carbon, C8hydrogen, H13oxygen, O5nitrogen, N)n ( ) is a long-chain polymer of N-Acetylglucosamine, ''N''-acetylglucosamine, an amide derivative of glucose. Chitin is the second most abundant polysaccharide in nature (behind only cell ...
. In pterygotids, the outer surface of the exoskeletons, ranging in size from small to gigantic, was composed of semilunar scales. The chelicerate body is divided into two tagmata (sections); the frontal
prosoma The cephalothorax, also called prosoma in some groups, is a tagma of various arthropods, comprising the head and the thorax fused together, as distinct from the abdomen behind. (The terms ''prosoma'' and ''opisthosoma'' are equivalent to ''cepha ...
(head) and posterior
opisthosoma The opisthosoma is the posterior part of the body in some arthropods, behind the prosoma ( cephalothorax). It is a distinctive feature of the subphylum Chelicerata (arachnids, horseshoe crabs and others). Although it is similar in most respects ...
(abdomen). The appendages were attached to the prosoma, and were characterized in pterygotids by being small and slender and lacking spines. The
telson The telson () is the hindmost division of the body of an arthropod. Depending on the definition, the telson is either considered to be the final segment (biology), segment of the arthropod body, or an additional division that is not a true segm ...
(the posteriormost division of the body) was expanded and flattened with a small median keel. The posterior margin (tip) of the telson forms a short spine in some genera (''
Pterygotus ''Pterygotus'' is an extinct genus of giant predatory eurypterid, a group of extinct aquatic arthropods. Fossils of ''Pterygotus'' have been discovered in deposits ranging in age from Middle Silurian to Late Devonian, and have been referred to s ...
'' and ''
Acutiramus ''Acutiramus'' is a genus of giant predatory eurypterid, an extinct group of aquatic arthropods. Fossils of ''Acutiramus'' have been discovered in deposits of Late Silurian to Early Devonian age. Eight species have been described, five from Nor ...
'') and is indented (giving a bilobed appearance) in others (''
Erettopterus ''Erettopterus'' is a genus of large predatory eurypterid, an extinct group of aquatic arthropods. Fossils of ''Erettopterus'' have been discovered in deposits ranging from Early Silurian (the Rhuddanian age) to the Early Devonian (the Lochkov ...
''). Like other chelicerates, pterygotids possessed
chelicerae The chelicerae () are the arthropod mouthparts, mouthparts of the subphylum Chelicerata, an arthropod group that includes arachnids, horseshoe crabs, and sea spiders. Commonly referred to as "jaws", chelicerae may be shaped as either articulated ...
. These appendages are the only ones that appear before the mouth and take the form of small pincers used to feed in all other eurypterid groups. In the pterygotids, the chelicerae were large and long, with strong well developed teeth on specialised
chelae A chela ()also called a claw, nipper, or pinceris a pincer-shaped organ at the end of certain limbs of some arthropods. The name comes from Ancient Greek , through Neo-Latin '. The plural form is chelae. Legs bearing a chela are called chelipeds ...
(claws). These specialised chelicerae, likely used for prey capture but differing in the exact role from genus to genus, are also the primary feature that distinguishes members of the group from eurypterids of the other pterygotioid families,
Slimonidae Slimonidae (the name deriving from the type genus ''Slimonia'', which is named in honor of Welsh fossil collector and surgeon Robert Slimon) is a family of eurypterids, an extinct group of aquatic arthropods. Slimonids were members of the super ...
and
Hughmilleriidae Hughmilleriidae (the name deriving from the type genus ''Hughmilleria'', which is named in honor of Scottish geologist Hugh Miller) is a family of eurypterids, an extinct group of aquatic arthropods. The hughmilleriids were the most basal member ...
, and other eurypterids in general.


History of research

Due to their unique features within the Eurypterida, the Pterygotidae has attracted a lot of attention ever since their discovery. The first fossils found, discovered by quarrymen in
Scotland Scotland is a Countries of the United Kingdom, country that is part of the United Kingdom. It contains nearly one-third of the United Kingdom's land area, consisting of the northern part of the island of Great Britain and more than 790 adjac ...
, were referred to as "
Seraph A seraph ( ; pl.: ) is a celestial or heavenly being originating in Ancient Judaism. The term plays a role in subsequent Judaism, Islam and Christianity. Tradition places seraphim in the highest rank in Christian angelology and in the fif ...
ims" by the quarrymen. When describing ''Pterygotus'' itself in 1839,
Louis Agassiz Jean Louis Rodolphe Agassiz ( ; ) FRS (For) FRSE (May 28, 1807 – December 14, 1873) was a Swiss-born American biologist and geologist who is recognized as a scholar of Earth's natural history. Spending his early life in Switzerland, he recei ...
first thought the fossils represented remains of fish, with the name meaning "winged one", and only recognized their nature as arthropod remains five years later in 1844. By 1859, 10 species (many of which would later be reassigned) had been assigned to ''Pterygotus''.
John William Salter John William Salter (15 December 1820 – 2 December 1869) was an English naturalist, geologist, and palaeontologist. Salter was apprenticed in 1835 to James De Carle Sowerby, and was engaged in drawing and engraving the plates for Sowerby' ...
recognized that it was possible to divide ''Pterygotus'' based on the morphology of the telsons of the species that had been assigned to it. He divided ''Pterygotus'' into subgenera, erecting ''Pterygotus'' (''
Erettopterus ''Erettopterus'' is a genus of large predatory eurypterid, an extinct group of aquatic arthropods. Fossils of ''Erettopterus'' have been discovered in deposits ranging from Early Silurian (the Rhuddanian age) to the Early Devonian (the Lochkov ...
'') for species with a bilobed telson. The family Pterygotidae was erected in 1912 by
John Mason Clarke John Mason Clarke (April 15, 1857 – May 29, 1925) was an American teacher, geologist and paleontologist. __TOC__ Early career Born in Canandaigua, New York, the fifth of six children of Noah Turner Clarke and Laura Mason Merrill, he attended ...
&
Rudolf Ruedemann Rudolf Ruedemann (October 16, 1864–June 18, 1956) was a German American paleontologist, widely known as an expert in graptolites, enigmatic fossil animals. He worked at the New York State Museum for over 40 years, including a decade as State ...
to constitute a group for the genera ''Pterygotus'', ''
Slimonia ''Slimonia'' is a genus of eurypterid, an extinct group of aquatic arthropods. Fossils of ''Slimonia'' have been discovered in deposits of Silurian age in South America and Europe. Classified as part of the family Slimonidae alongside the relat ...
'', '' Hastimima'' and ''
Hughmilleria ''Hughmilleria'' is a genus of eurypterid, an extinct group of aquatic arthropods. Fossils of ''Hughmilleria'' have been discovered in deposits of the Silurian age in China and the United States. Classified as part of the basal family Hughmille ...
''. ''Pterygotus'' would also designated as containing two "
subgenera In biology, a subgenus ( subgenera) is a taxonomic rank directly below genus. In the International Code of Zoological Nomenclature, a subgeneric name can be used independently or included in a species name, in parentheses, placed between the ge ...
", ''Pterygotus'' (''Curviramus'') and ''Pterygotus'' (''
Acutiramus ''Acutiramus'' is a genus of giant predatory eurypterid, an extinct group of aquatic arthropods. Fossils of ''Acutiramus'' have been discovered in deposits of Late Silurian to Early Devonian age. Eight species have been described, five from Nor ...
'') in 1935, differentiated by the curvature of the denticles (teeth) of the chelicerae. The same year (1935), Leif Størmer named a new pterygotid genus, ''
Grossopterus ''Grossopterus'' is a genus of prehistoric eurypterid classified as part of the family Waeringopteridae. The genus contains two species, ''G. inexpectans'' from Gilboa, United States and ''G. overathi'' from Overath, Germany.Dunlop, J. A., Penn ...
'', and split ''Pterygotus'' into two other subgenera, ''Pterygotus'' (''Pterygotus'') and ''Pterygotus'' (''Erettopterus''), designating ''Pterygotus'' (''Curviramus'') as a junior synonym of ''Pterygotus'' (''Pterygotus'') and not recognizing ''Pterygotus'' (''Acutiramus''). A division into three subgenera of ''Pterygotus'' was proposed by Ferdinand Prantl and Alois Přibyl in 1948, retaining ''P.'' (''Erettopterus'') and ''P.'' (''Pterygotus'') but also restoring ''P.'' (''Acutiramus'') to subgenus level. Erik N. Kjellesvig-Waering emended the family in 1951, when the genera ''Hastimima, Hughmilleria'', ''Grossopterus'' and ''Slimonia'' were referred to their own family, the
Hughmilleriidae Hughmilleriidae (the name deriving from the type genus ''Hughmilleria'', which is named in honor of Scottish geologist Hugh Miller) is a family of eurypterids, an extinct group of aquatic arthropods. The hughmilleriids were the most basal member ...
, which left ''Pterygotus'' as the only genus within the Pterygotidae. In 1961, Kjellesvig-Waering raised ''Erettopterus'' to the level of its own genus, recognizing two subgenera of ''Pterygotus''; ''P.'' (''Pterygotus'') and ''P.'' (''Acutiramus''), as well as two subgenera of ''Erettopterus''; ''E.'' (''Erettopterus'') and ''E.'' (''Truncatiramus''). Kjellesvig-Waering placed the primary taxonomical value on the morphology of the telson, considering potential differences in the chelicerae and
metastoma The metastoma is a ventral single plate located in the opisthosoma of non-arachnid dekatriatan chelicerates such as eurypterids, chasmataspidids and the genus '' Houia''. The metastoma located between the base of 6th prosomal appendage pair and ...
(a large plate that is part of the abdomen) to be secondary in importance. ''Jaekelopterus'', previously designated as a species of ''Pterygotus'', was separated into a distinct genus in 1964 based on the supposed different segmentation of the genital appendage. These supposed differences would later turn out to be false, but briefly prompted ''Jaekelopterus'' to be classified within a family of its own, the "Jaekelopteridae". The error with the genital appendage was later discovered and rectified, making ''Jaekelopterus'' a member of the Pterygotidae once more. In 1974, Størmer raised the ''Pterygotus'' subgenera ''Acutiramus'' and ''Truncatiramus'' to the level of separate genera. ''Truncatiramus'' has later been recognized as representing a synonym of ''Erettopterus''.Dunlop, J. A., Penney, D. & Jekel, D. 2018
A summary list of fossil spiders and their relatives
In World Spider Catalog. Natural History Museum Bern
In 1986, Paul Selden examined the fossil material of the enigmatic arthropod ''
Necrogammarus ''Necrogammarus salweyi'' is the binomial name applied to an arthropod fossil discovered in Herefordshire, England. The fossil represents a fragmentary section of the underside and an Arthropod leg, appendage of a Pterygotidae, pterygotid eurypt ...
'' and concluded that the specimen represents the infracapitulum and attached
palp Pedipalps (commonly shortened to palps or palpi) are the secondary pair of forward appendages among chelicerates – a group of arthropods including spiders, scorpions, horseshoe crabs, and sea spiders. The pedipalps are lateral to the chelicera ...
of a large pterygotid. The fossil likely belongs to either '' Erettopterus marstoni'' or '' Pterygotus arcuatus'', both found in the same locality, but the lack of key diagnostic features in the ''Necrogammarus'' remains makes assignment to either impossible, and therefore, ''Necrogammarus'' is considered an unspecified pterygotid. In 2009, ''Pterygotus ventricosus'' was recognized as being distinct from, and far more basal than, other species in its genus and was thus named as the type species of a new genus, ''
Ciurcopterus ''Ciurcopterus'' is a genus of eurypterid, an extinct group of aquatic arthropods. Fossils of ''Ciurcopterus'' have been discovered in deposits of Late Silurian age in North America. Formerly classified as part of the family Pterygotidae, but pl ...
''. Studies of specimens referred to this genus resolved long-standing contentiousness about the precise phylogenetic position of the Pterygotidae, providing evidence in the form of shared characteristics that ''Slimonia'', not '' Herefordopterus'' or ''Hughmilleria'' as previously thought, was the closest sister taxon of the group.


Evolutionary history

The pterygotids were one of the most successful eurypterid groups, with fossilised remains having been discovered on all continents except Antarctica. They are the only eurypterid group with a
cosmopolitan distribution In biogeography, a cosmopolitan distribution is the range of a taxon that extends across most or all of the surface of the Earth, in appropriate habitats; most cosmopolitan species are known to be highly adaptable to a range of climatic and en ...
. Their remains range in age from 428 to 372 million years old (for a total temporal range of approximately 56 million years), reaching their greatest diversity during the Late Silurian, a period in time when other eurypterid groups became increasingly diverse as well. The enlargement and specialisation of the chelicerae within the Pterygotidae has been recognised as one of the two most striking evolutionary innovations within the Eurypterida, besides the transformation of the most posterior prosomal appendage into a swimming paddle (a trait seen in all eurypterids in the Eurypterina suborder). The most primitive and basal pterygotid, ''Ciurcopterus'', preserves a mixture of characteristics that are reminiscent of ''Slimonia'', which is often interpreted as a sister-taxon of the Pterygotidae, as well as more derived pterygotids. The appendages were similar to those of ''Slimonia'' but the carapace clearly belonged to a pterygotid, further suggesting a close relationship between the Pterygotidae and the
Slimonidae Slimonidae (the name deriving from the type genus ''Slimonia'', which is named in honor of Welsh fossil collector and surgeon Robert Slimon) is a family of eurypterids, an extinct group of aquatic arthropods. Slimonids were members of the super ...
within the
Pterygotioidea Pterygotioidea (the name deriving from the type genus ''Pterygotus'', meaning "winged one") is a superfamily (taxonomy), superfamily of eurypterids, an extinct group of aquatic arthropods. Pterygotioids were the most Derived trait, derived membe ...
superfamily.


Potential influence in vertebrate evolution

Alfred S. Romer suggested in 1933 that early vertebrate evolution might have been heavily influenced by pterygotid predation. Early vertebrates of the
Late Silurian The Silurian ( ) is a geologic period and system spanning 23.5 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the third and shortest period of t ...
and
Devonian The Devonian ( ) is a period (geology), geologic period and system (stratigraphy), system of the Paleozoic era (geology), era during the Phanerozoic eon (geology), eon, spanning 60.3 million years from the end of the preceding Silurian per ...
are often heavily armored, and it is likely that this represents an ancestral vertebrate trait that was later lost or reduced, rather than something that evolved separately in several groups at the same time. Some researchers have suggested that the armor was to protect against hitting rocky surfaces in fast flowing streams, but Romer pointed out that there is no such armor protection in modern fish that live in that type of environment. Instead, Romer stated that the only reasonable explanation for the armor was as "a protection against living enemies". With most of the early vertebrates of the Silurian being just a few decimetres in length and often occurring together with pterygotid eurypterids in
freshwater Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. The term excludes seawater and brackish water, but it does include non-salty mi ...
environments, they would seem to represent appropriate prey for the pterygotids, which were large predators with grasping claws. There are few other animals that would present appropriate prey and there are virtually no other predators than the pterygotids that would warrant the evolution of armored protection in their prey. The pterygotids reached their maximum size and number in the Late Silurian and Early Devonian, after which they saw rapid decline during the Devonian. This decline occurred at around the same time as there was an increase in unarmored vertebrates as well as a growth in fish size and the increased migration of fish into marine environments. The Devonian would also see the evolution of significantly faster-moving fish and the evolution of proper jaws. These adaptations, potentially a result of pterygotid predation, would have significantly affected the likelihood of fish representing pterygotid prey and larger predatory fish may even have begun preying on pterygotids and other eurypterids, contributing to their decline and extinction. The arguments of Romer were based on evolutionary trends in both groups and the fossil co-occurrences of both groups but he did not present a detailed analysis. The groups do frequently occur together, with pterygotids present at more than two-thirds of fossil localities where eurypterids and fish are recorded together. There is also a recorded increase in fish diversity at the same time as the eurypterids began to decline in the early Devonian, but available data does not support any direct competitive replacement. Though the pterygotids would be extinct at that point, both fish and eurypterids would decline in the Middle Devonian only to peak again in the late Devonian and to begin another decline in the
Permian The Permian ( ) is a geologic period and System (stratigraphy), stratigraphic system which spans 47 million years, from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.902 Mya. It is the s ...
. Detailed analyses have failed to find any correlation between the extinction of the pterygotids and the diversification of the vertebrates.


Paleobiology


Cheliceral claws

The function of pterygotid chelicerae was likely the same as the chelicerae of other eurypterids as well as those of other arthropods, such as
crustacean Crustaceans (from Latin meaning: "those with shells" or "crusted ones") are invertebrate animals that constitute one group of arthropods that are traditionally a part of the subphylum Crustacea (), a large, diverse group of mainly aquatic arthrop ...
s and
xiphosura Xiphosura (; , in reference to its sword-like telson) is an order of arthropods related to arachnids. They are more commonly known as horseshoe crabs (a name applied more specifically to the only extant family, Horseshoe crab, Limulidae). They fir ...
ns; the capture and cutting of food into smaller pieces and transport of food into the mouth as well as defense. Though most other eurypterid families had simple pincers, the Pterygotidae is the only eurypterid family to possess enlarged and robust chelicerae with claws and teeth, showing unique adaptations to defense and/or prey capture. The chelicerae were composed of several joints, though the exact number is somewhat controversial with some researchers stating three, others four and some claiming that the number of joints varies between three and five depending on the species and genus in question (''Pterygotus'' would have three joints and ''Erettopterus'' five). The most common interpretation historically was that the number of joints were three, with a long basal joint followed by two smaller distal joints with teeth. More modern research on very complete specimens of ''Acutiramus'' and ''Erettopterus'' has revealed that the actual count appears to be four joints. Additionally, a three-joint anatomy would have placed the claws at the end of appendages that would essentially have been rigid stalks, rendering their function useless. To have the necessary mobility, the pterygotid chelicerae would have had to have been composed of four joints. The first joint of the chelicerae, where it connects to the epistoma (a plate located on the
prosoma The cephalothorax, also called prosoma in some groups, is a tagma of various arthropods, comprising the head and the thorax fused together, as distinct from the abdomen behind. (The terms ''prosoma'' and ''opisthosoma'' are equivalent to ''cepha ...
, or "head"), would have been capable of turning the entire appendage in a twisting way, which has led researchers to conclude that the function of the chelicerae would not have been only, or even primarily, for defense but rather to capture and convey food to the mouth. When captured, prey would need to be broken into smaller pieces to be able to fit into the mouth; eurypterid mouths were even less adapted to devour large pieces than mouths of modern
crab Crabs are decapod crustaceans of the infraorder Brachyura (meaning "short tailed" in Greek language, Greek), which typically have a very short projecting tail-like abdomen#Arthropoda, abdomen, usually hidden entirely under the Thorax (arthropo ...
s are. The eurypterid walking appendages could not cut, transport or grasp anything, and as such this would also be done with the chelicerae. In crabs, the claws tear food apart and then transport the smaller pieces to the mouth. Based on the feeding process seen in modern arthropods with chelicerae, one of the claws would hold the prey while the other would cut off pieces and transport it to the mouth with continuous and simple movements.


Telson

The large and flattened pterygotid telson is a distinctive feature of the group that is only shared by the closely related ''Slimonia'' and by the derived
hibbertopterid Hibbertopteridae (the name deriving from the type genus ''Hibbertopterus'', meaning "Samuel Hibbert-Ware, Hibbert's wing") is a family of Eurypterid, eurypterids, an extinct group of aquatic Arthropod, arthropods. They were members of the superfam ...
''
Hibbertopterus ''Hibbertopterus'' is a genus of eurypterid, a group of extinct marine arthropods. Fossils of ''Hibbertopterus'' have been discovered in deposits ranging from the Devonian period in Belgium, Scotland and the United States to the Carboniferous pe ...
'' and mycteroptid '' Hastimima'', where a flattened telson had
convergently evolved Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last comm ...
. The telson is in general flat but with a raised thin median keel. The posterior margin (tip) of the telson form a short spine in some genera, such as ''Pterygotus'' and ''Acutiramus'', and is indented (giving a bilobed appearance) in ''Erettopterus''. The function of these specialised telsons has historically been controversial and disputed. Erik N. Kjellesvig-Waering compared the pterygotid telson to the large tail fluke of
whale Whales are a widely distributed and diverse group of fully Aquatic animal, aquatic placental mammal, placental marine mammals. As an informal and Colloquialism, colloquial grouping, they correspond to large members of the infraorder Cetacea ...
s in 1964. The pterygotids were hypothesized to have moved by undulating the entire
opisthosoma The opisthosoma is the posterior part of the body in some arthropods, behind the prosoma ( cephalothorax). It is a distinctive feature of the subphylum Chelicerata (arachnids, horseshoe crabs and others). Although it is similar in most respects ...
(the large posterior section of the body) by moving the abdominal plates, as such undulations of the opisthosoma and telson would have acted as the propulsive method of the animal, rendering the swimming legs used by other eurypterid groups useless. What is known of eurypterid anatomy contradicts the undulation hypothesis simply because eurypterid bodies were likely very stiff. The body segments were nearly equal in width and thickness with little difference in size between segments directly adjoining to each other, while there is no evidence for any sort of tapering or other mechanism that would have increased flexibility. Any flexing of the body would require muscular contractions, but no major
apodeme An exoskeleton () . is a skeleton that is on the exterior of an animal in the form of hardened integument, which both supports the body's shape and protects the internal organs, in contrast to an internal endoskeleton (e.g. that of a human) wh ...
s (internal ridges of the exoskeleton that supports muscular attachments) or any muscle scars indicative of large opisthosomal muscles have been found. Instead, pterygotids were most likely propelled by the enlarged and flattened paddlelike sixth pair of prosomal appendages, like other swimming eurypterids. An alternate hypothesis first proposed by C. D. Waterston in 1979 postulates that the median keel and the telson at large was used to steer the body, working more like a vertical and horizontal rudder than a tail fluke. Calculations and the creation of models of plaster allowed Plotnick ''et al.'' (1988) to determine that the design of the pterygotid telson could functionally work as a rudder, which would have enabled the pterygotids to be agile animals capable of quick turns when chasing after prey.


Gigantism

The Pterygotidae includes the largest known arthropods to have ever lived, with several species surpassing two metres in length (such as ''
Jaekelopterus rhenaniae ''Jaekelopterus'' is a genus of predatory eurypterid, a group of extinct aquatic arthropods. Fossils of ''Jaekelopterus'' have been discovered in deposits of Early Devonian age, from the Pragian and Emsian stages. There are two known species: t ...
'' at 2.5 metres and '' Acutiramus bohemicus'' at 2.1 metres). There are several known factors that restrict the size arthropods are able to grow to. These factors include respiration, the energy it costs to moult, locomotion and the properties of the exoskeleton. Except the cheliceral claws, which are robust and heavily sclerotized, a majority of fossilized large pterygotid body segments are unmineralized and thin. Even the plates that form the surface of the abdominal segments, the stergites and
sternites The sternum (: sterna) is the ventral portion of a segment of an arthropod thorax or abdomen. In insects, the sterna are usually single, large sclerites, and external. However, they can sometimes be divided in two or more, in which case the sub ...
, are preserved as paper-thin compressions which suggests that pterygotids were very light-weight in construction. Similar adaptations have been observed in other prehistoric giant arthropods, such as ''
Arthropleura ''Arthropleura'', from Ancient Greek ἄρθρον (''árthron''), meaning "joint", and πλευρά (''pleurá''), meaning "rib", is an extinct genus of massive myriapoda, myriapod that lived in what is now Europe and North America around 344 t ...
'', and may be vital to the evolution of arthropod gigantism as a light-weight build decreases the influence of size-restricting factors. Though they were the largest arthropods known to have ever existed, the light-weight build of the pterygotids means that they are unlikely to have been the heaviest. Giant eurypterids of other lineages, notably the deep-bodied walking forms of the Hibbertopteridae, such as the almost 2 metre long ''Hibbertopterus'', might have rivalled the pterygotids in weight, if not surpassed them.


Classification


Taxonomy

Since its creation by
John Mason Clarke John Mason Clarke (April 15, 1857 – May 29, 1925) was an American teacher, geologist and paleontologist. __TOC__ Early career Born in Canandaigua, New York, the fifth of six children of Noah Turner Clarke and Laura Mason Merrill, he attended ...
&
Rudolf Ruedemann Rudolf Ruedemann (October 16, 1864–June 18, 1956) was a German American paleontologist, widely known as an expert in graptolites, enigmatic fossil animals. He worked at the New York State Museum for over 40 years, including a decade as State ...
in 1912, the phylogenetic status of the Pterygotidae has changed several times. Leif Størmer considered the group to represent a family within the eurypterid superfamily Eurypteracea. In 1962,
Nestor Ivanovich Novojilov Nestor Ivanovich Novozhilov was a Soviet paleontologist. In 1948, Novozhilov described a pliosaur specimen discovered on the banks of Russia's Volga Riveras a new species, ''Pliosaurus rossicus''. The specimen, while large, was damaged during the e ...
raised the groups in question to subordinal and superfamily status, Eurypteracea becoming the suborder
Eurypterina Eurypterina is one of two suborders of eurypterids, an extinct group of chelicerate arthropods commonly known as "sea scorpions". Eurypterine eurypterids are sometimes informally known as "swimming eurypterids". They are known from fossil depos ...
and creating the superfamily
Pterygotioidea Pterygotioidea (the name deriving from the type genus ''Pterygotus'', meaning "winged one") is a superfamily (taxonomy), superfamily of eurypterids, an extinct group of aquatic arthropods. Pterygotioids were the most Derived trait, derived membe ...
. Both Størmer (in 1974) and Erik N. Kjellesvig-Waering (in 1964) would come to consider the pterygotids as distinctive enough, due to their uniquely enlarged chelicerae, to warrant the status of a separate suborder, which was dubbed the "Pterygotina". More modern cladistics and phylogenetic analyses does not support the classification of the pterygotids as a suborder, but classifies them within the superfamily Pterygotioidea as the most derived members of the suborder Eurypterina. The cladogram below is simplified from a study by Tetlie (2007), showcasing the derived position of the pterygotids within Eurypterina.


Internal phylogeny

The clade Pterygotidae is among the best supported within the Eurypterida. Relationships within it has historically been difficult to resolve due to wrong interpretations of genital appendage of ''Jaekelopterus'', and the consequential disturbance of character states historically interpreted as primitive and derived within the group when the error was solved. Subsequent descriptions and redescriptions have ensured that the phylogeny of the clade is rather robust at the genus level. Yet, a comprehensive species-level phylogenetic analysis has proven impossible due to the large amount of species based on scant and fragmentary fossilised material. The genus ''Slimonia'' is thought to represent the sister group to the pterygotids. The cladogram below is based on the nine best-known pterygotid species and two outgroup taxa (''Slimonia acuminata'' and '' Hughmilleria socialis''). The cladogram also contains the primary unifying characteristics for the various clades, as well as the maximum sizes reached by the species in question, which have been suggested to possibly have been an evolutionary trait of the group per
Cope's rule Cope's rule, named after American paleontologist Edward Drinker Cope, postulates that population lineages tend to increase in body size over evolutionary time. It was never actually stated by Cope, although he favoured the occurrence of linear e ...
("phyletic gigantism").


Paleoecology

Traditionally interpreted as visual and active predators as a group, recent studies on the cheliceral morphology and
visual acuity Visual acuity (VA) commonly refers to the clarity of visual perception, vision, but technically rates an animal's ability to recognize small details with precision. Visual acuity depends on optical and neural factors. Optical factors of the eye ...
of the pterygotid eurypterids have revealed that it is possible to separate them into distinct ecological groups. The primary method for determining visual acuity in arthropods is by determining the number of lenses in their
compound eyes A compound eye is a visual organ found in arthropods such as insects and crustaceans. It may consist of thousands of ommatidia, which are tiny independent photoreception units that consist of a cornea, lens, and photoreceptor cells which distin ...
and the interommatidial angle (abbreviated as IOA and referring to the angle between the optical axes of the adjacent lenses). The IOA is especially important as it can be used to distinguish different ecological roles in arthropods, being low in active predators. Despite morphological similarities within the group, the ecology of the pterygotids differed greatly from genus to genus. The vision of ''Erettopterus'' was similar to that of the more basal pterygotoid ''Slimonia'' and more acute than the more derived ''Acutiramus'', though it was not as acute as the vision of the
apex predator An apex predator, also known as a top predator or superpredator, is a predator at the top of a food chain, without natural predators of its own. Apex predators are usually defined in terms of trophic dynamics, meaning that they occupy the hig ...
s ''Jaekelopterus'' and ''Pterygotus'' or modern actively predatory arthropods. Additionally, the chelicerae of ''Erettopterus'' suggest that it was a generalised feeder rather than a highly specialised predator. The claws in ''Erettopterus'' are enlarged, as in other pterygotids, though the differentiated denticles and paired distal teeth mean that they were likely not used for specialised feeding, but solely for grasping. Though the number of lenses in its compound eyes is comparable to more derived members of the group, its morphology suggests that it was not as active, nor as specialised as ''Pterygotus'' or ''Jaekelopterus''. The eyes of ''Acutiramus'' were low in visual acuity (with few lenses in the compound eyes and high IOA values), inconsistent with the traditionally assumed pterygotid lifestyle of "active and high-level visual predators". The IOA values of ''Acutiramus'' changed during ontogeny but in a way opposite to other pterygotids. Vision becomes less acute in larger specimens, whilst vision tends to get more acute in adults in other genera, such as in ''Jaekelopterus''. Pterygotids may thus have been almost equally visually acute early in their life cycle, becoming more differentiated during growth. The chelicerae of ''Acutiramus'' likely served as slicing or shearing devices, adding to the evidence that it would have occupied a distinct
ecological niche In ecology, a niche is the match of a species to a specific environmental condition. Three variants of ecological niche are described by It describes how an organism or population responds to the distribution of Resource (biology), resources an ...
. A significantly less active predator, ''Acutiramus'' might have been a scavenger or ambush predator, feeding on soft-bodied animals. Both ''Jaekelopterus'' and ''Pterygotus'' have a very high visual acuity, which researchers could determine by observing low IOA values and large numbers of lenses in their compound eyes. The chelicerae of these genera were enlarged, robust and possessed a curved free ramus and denticles of different lengths and sizes, all adaptations that correspond to strong puncturing and grasping abilities in extant
scorpion Scorpions are predatory arachnids of the Order (biology), order Scorpiones. They have eight legs and are easily recognized by a pair of Chela (organ), grasping pincers and a narrow, segmented tail, often carried in a characteristic forward cur ...
s and crustaceans. These genera likely represented active and visual apex predators.


See also

*
List of eurypterid genera This list of eurypterid genera is a comprehensive listing of all genera that have ever been included in the order Eurypterida, excluding purely vernacular terms. The list includes all commonly accepted genera, but also genera that are now consider ...
*
Timeline of eurypterid research This timeline of eurypterid research is a chronologically ordered list of important fossil discoveries, controversies of interpretation, and Biological taxonomy, taxonomic revisions of eurypterids, a group of extinct aquatic arthropods closely re ...
*
Hughmilleriidae Hughmilleriidae (the name deriving from the type genus ''Hughmilleria'', which is named in honor of Scottish geologist Hugh Miller) is a family of eurypterids, an extinct group of aquatic arthropods. The hughmilleriids were the most basal member ...
*
Slimonidae Slimonidae (the name deriving from the type genus ''Slimonia'', which is named in honor of Welsh fossil collector and surgeon Robert Slimon) is a family of eurypterids, an extinct group of aquatic arthropods. Slimonids were members of the super ...


References

{{Taxonbar, from=Q7257012 Pterygotioidea Silurian animals Devonian animals Llandovery first appearances Early Devonian extinctions Prehistoric arthropod families